PyramidHead
Contributor
P1. If my brain were split in two and the halves were successfully implanted into two new bodies, each body would have its own independent stream of consciousness simultaneously, with no access to that of the other.
P2. If the two halves were subsequently recombined into a whole and successfully implanted in a single body, this body would have a single stream of consciousness with full access to the memories of what both previous bodies experienced.
P3. Although each original half could only experience its own stream of consciousness while divided, the resulting unified brain would remember both streams of consciousness as if they were events from its own past.
P4. Unlike the ordinary process of recalling past events, the experiences of each original half could not be ordered in time with respect to each other by the resulting unified brain; there is no answer to the question of which half was experienced "first" by the single stream of consciousness generated by the unified brain.
P5. Since the now-unified brain has unbroken psychological connections to the experiences of both halves during the split as well as its own experiences before the split, it is the same person as each half during the split and itself before the split.
P6. During the split, each half of the brain was the same person as the other half, the same person as the brain prior to the split, and the same person as the recombined brain in the future, even if the recombination had not yet taken place or would never take place. (Transitive property of identity.)
C1. It is possible to be the same person as someone who exists simultaneously and has different brain matter.
C2. If all human brains were successfully merged together, the resulting unified brain would have access to all the memories of each connected brain in the same way that the unified single brain had access to the memories of each half during the split.
C3. This unified brain would have no way of sorting the individual stories from the connected brains relative to each other, as they occurred simultaneously before the brains were merged.
C4. Since the unified brain would have unbroken psychological access to each connected brain, it is really the same person as each of those brains.
C5. If the individual brains are all the same person as the unified brain, they are all the same person as each other, and would be the same person even if they had not been merged.
C6. We are all actually the same person.
I anticipate objections between P5 and P6. It is unproblematic to say that a single brain with one stream of consciousness, remembering everything that happened to its brain, is a single person. This is what we say about ourselves and others all the time. We also accept that numerical identity is transitive and indivisible. That is, if a and b are identical to c, a is numerically identical to b (i.e. they are all just one actual thing). Further, if a, b, and c are all numerically identical at time t, they are numerically identical at times t-1 and t+1. In order to deny P6, one must assert one of the following things:
1) The person who exists after the brain is split and recombined is not the same person as the original halves of the brain, but it is the same person as the original brain before the split.
2) The person who exists after the brain is split and recombined is not even the same person as the original brain before the split.
Option 1 is harder to defend than 2, because it implies that numerically the same thing existed at time t (before the split) and time t+2 (after the recombination), but not at time t+1 (during the split). This is an exception to how we normally think about identity that needs its own justification. Option 2 commits one to the view that despite having direct psychological connection/access to all of your memories and experiences, you are not the same person as any time-slice of your brain during its existence. This conclusion is tantamount to denying that persons exist at all, and is not far from saying we are all the same person. In either case, the number of persons is not greater than 1, and it is false to say you are a different person from anyone else. However, taking option 2 necessitates abandoning the powerful intuition that you are a person, period. To reconcile option 2 with the conclusion of my argument, I will say this: to the extent that you are the same person as the person who started reading this sentence from where you are sitting, you are the same person as me and any other being with consciousness.
P2. If the two halves were subsequently recombined into a whole and successfully implanted in a single body, this body would have a single stream of consciousness with full access to the memories of what both previous bodies experienced.
P3. Although each original half could only experience its own stream of consciousness while divided, the resulting unified brain would remember both streams of consciousness as if they were events from its own past.
P4. Unlike the ordinary process of recalling past events, the experiences of each original half could not be ordered in time with respect to each other by the resulting unified brain; there is no answer to the question of which half was experienced "first" by the single stream of consciousness generated by the unified brain.
P5. Since the now-unified brain has unbroken psychological connections to the experiences of both halves during the split as well as its own experiences before the split, it is the same person as each half during the split and itself before the split.
P6. During the split, each half of the brain was the same person as the other half, the same person as the brain prior to the split, and the same person as the recombined brain in the future, even if the recombination had not yet taken place or would never take place. (Transitive property of identity.)
C1. It is possible to be the same person as someone who exists simultaneously and has different brain matter.
C2. If all human brains were successfully merged together, the resulting unified brain would have access to all the memories of each connected brain in the same way that the unified single brain had access to the memories of each half during the split.
C3. This unified brain would have no way of sorting the individual stories from the connected brains relative to each other, as they occurred simultaneously before the brains were merged.
C4. Since the unified brain would have unbroken psychological access to each connected brain, it is really the same person as each of those brains.
C5. If the individual brains are all the same person as the unified brain, they are all the same person as each other, and would be the same person even if they had not been merged.
C6. We are all actually the same person.
I anticipate objections between P5 and P6. It is unproblematic to say that a single brain with one stream of consciousness, remembering everything that happened to its brain, is a single person. This is what we say about ourselves and others all the time. We also accept that numerical identity is transitive and indivisible. That is, if a and b are identical to c, a is numerically identical to b (i.e. they are all just one actual thing). Further, if a, b, and c are all numerically identical at time t, they are numerically identical at times t-1 and t+1. In order to deny P6, one must assert one of the following things:
1) The person who exists after the brain is split and recombined is not the same person as the original halves of the brain, but it is the same person as the original brain before the split.
2) The person who exists after the brain is split and recombined is not even the same person as the original brain before the split.
Option 1 is harder to defend than 2, because it implies that numerically the same thing existed at time t (before the split) and time t+2 (after the recombination), but not at time t+1 (during the split). This is an exception to how we normally think about identity that needs its own justification. Option 2 commits one to the view that despite having direct psychological connection/access to all of your memories and experiences, you are not the same person as any time-slice of your brain during its existence. This conclusion is tantamount to denying that persons exist at all, and is not far from saying we are all the same person. In either case, the number of persons is not greater than 1, and it is false to say you are a different person from anyone else. However, taking option 2 necessitates abandoning the powerful intuition that you are a person, period. To reconcile option 2 with the conclusion of my argument, I will say this: to the extent that you are the same person as the person who started reading this sentence from where you are sitting, you are the same person as me and any other being with consciousness.
